The present invention relates generally to teletext communications systems and particularly to the addressability of service levels, also called tiers, in a teletext data base. The present invention provides dynamic addressability to permit the system operator to adjust the tiers to which an individual, remotely located subscriber can have access and without requiring changes at the remote subscriber location.
Teletext as a television based communication technique for the transmission of text and graphics information has been an acknowledged, operational technique for some years now. Originally, systems implemented a teletext feature by transmitting text and graphics material in digitally encoded form on only a few lines of a multi-line system. For example, the British teletext system transmitted such material only on lines 17, 18, 330 and 331 of their 625 line system. At each receiving end, a television receiver is equipped with a teletext decoder to process the data and form a display on the viewing screen.
In the typical teletext system, the broadcast teletext information is formatted ina group of cyclically transmitted rows of data, twenty four rows comprising a page of information. In many of the systems currently in use, each group of one hundred pages is separately identified as a magazine. Each row of information, which represents up to 40 characters of text or graphic data, is serially transmitted in digitally encoded form on one horizontal line. In early teletext systems this was sent only in the vertical blanking interval so that the data transmission rate was in the neighborhood of only 2 rows per field or four pages per second. With such systems, it was customary to limit the cyclically transmitted data to 200 pages so as to provide reasonable access time.
With the advent of cable television systems it has become practical to transmit full-field teletext, i.e. a system which does not limit its teletext data transmission to the vertical interval. A full field system thus can carry essentially only teletext data, together with supporting NTSC or other signals. In a full-field teletext system, transmission rates of 500 pages per second can be easily achieved with a 40 character, 24 row page display format. This allows an expanded data base of at least 5000 pages with an average access wait-time of about 5 seconds.
Typically each teletext decoder located at a subscriber station includes a data acquisition circuit responsive to user (subscriber) commands for acquiring a selected page of the recycling teletext information and for storing the twenty four rows of digitally encoded data comprising the page. The stored encoded data is then applied to a display generator which decodes the stored information for synthesizing R, G, B video character signals which are coupled to and drive the cathode ray tube (CRT) of the television receiver for producing a display corresponding to the stored data, the display comprising a page of twenty four rows of text or graphics with each row consisting of up to 40 characters.
In such a system, the first row (row 0) of each page is referred to as a page-header and includes an encoded magazine and page number as well as a row address. The remaining rows include only magazine and row address but no page number codes. Acquisition of a selected page of data is therefore achieved in the teletext decoder by initially matching the magazine and page number codes of a transmitted page-header row with user selected magazine and page number codes and then storing such matching page-header row together with the twenty three rows containing the corresponding magazine.
In this regard, it is to be appreciated that the data acquisition process cannot be completed until a match is achieved between the magazine and page number codes of a received page-header row and the magazine and page number codes selected by the user of the teletext decoder. The data acquisition time, i.e. the time between the selection by a viewer of a desired page and the presentation thereof on the screen of the receiver, is therefore dependent upon the relative position of the selected page in the cyclical transmission at the time the viewer enters the corresponding magazine and page number codes. This waiting period is appreciably shorter on the full-field system than in the previous systems.
In a modification to use a cable television system, it is desirable to present to all subscribers of the cable system a wide ranging data base divided into a number of service levels (tiers) covering various consumer interest areas, e.g., news, weather, sports, entertainment, professional information, home and auto repair, business information, television guides, advertising, health and medical information, etc. Subscribers pay a fee corresponding to the service level they want to decode, so it is desirable to be able to control separately each user's access to the various tiers.
One technique for accomplishing this selective viewing, or addressability of programming as it is commonly known, is described in U.S. Pat. No. 4,393,404 by Charles M. Cox and William Thomas, issued July 12, 1983, owned by Zenith Radio Corporation, the assignee of the present invention, the disclosure of which is incorporated herein by reference. In that technique, a novel data decoder is used to allow the subscriber who possesses it to view special services which are not available to the general receiving public. Although this technique is directed toward limited line teletext systems, it is illustrative of the typical scheme used to selectively address particular portions of the teletext data base.
There is a drawback to the type of addressing used in the above-cited example. Particularly, one must change the decoder of the subscriber station in order for that subscriber to alter his service tier access.
It would be desirable from both a cost effectiveness point of view and from a convenience point of view if the system operator could permit selected subscriber to gain access to additional tiers without the cost and trouble of modifying equipment already located at the remote subscriber station. Consequently, a provision in a teletext system for head-end (transmitting station) control of tier accessability would be beneficial if such accessibility control (addressing) were to have dynamic capabilities so as not to require reprogramming of the subscriber equipment or even any change in permanent coding status for that subscriber.
It is therefore a basic object of the present invention to provide dynamic addressability capabilities in an improved teletext system useful in full-field or other teletext systems, so as to allow selective controlled access to a tiered teletext data base for each subscriber of a cable television system.
It is a further object of the invention to allow additional accessibility to paying subscribers by redefining the range of material in any particular tier to which that subscriber already has access.
Another object of the invention is to provide for head-end, redefinition of the appropriate tier without any changes at the subscriber station.